There are a number of methods in which vibratory energy, such as ultrasound energy, can be used to produce images of objects. In the reflection method, an image is produced in which the brightness of each pixel is a function of the amplitude of the ultrasound reflected from the object to the receiver.
Ultrasonic transducers for medical applications are conventionally constructed from one or more piezoelectric elements sandwiched between a pair of electrodes. When an appropriate voltage pulse is applied, the piezoelectric element emits an ultrasonic pulse into the medium. Conversely, when an ultrasonic pulse strikes the piezoelectric element, the element produces a corresponding voltage across its electrodes. A number of such ultrasonic transducer constructions are disclosed in U.S. Pat. Nos. 4,217,684; 4,425,525; 4,441,503; 4,470,305 and 4,569,231, all of which are assigned to the instant assignee.
When used for ultrasound imaging, the transducer typically has a plurality of piezoelectric elements arranged in an array and driven with separate voltages. In phased-array imaging systems, transmit steering and focusing are accomplished by making the pulse signals transmitted by individual transducer elements arrive at the same time at a given point in space. By properly controlling the relative time delays of the applied voltages on each element, the ultrasonic waves produced by the piezoelectric elements can be made to combine to produce a net ultrasonic wave focused at a selected point. This focal point can be moved on each successive transmitter firing, so that the transmitted beams can be scanned across the object without moving the transducer.
Similar principles apply when the transducer is employed to receive the reflected sound. The voltages produced at the transducer elements in the array are individually delayed in time and then summed together such that the net signal is dominated by sound reflected from a single receive focal point in the subject. This summed receiver signal is often called the "beamsum".
If the medium is homogeneous and the velocity of sound is known, then the time it takes for a pulse to travel from a given transducer element to a point in space, or vice versa, is determined by simple geometry. Thus, to make all of the pulses arrive in coincidence, time delays are calculated to exactly compensate for geometric path length differences to each element.
For a wave at a single frequency f, it is well known that a shift in time .DELTA.t is equivalent to a shift in phase .DELTA..theta. through the relationship .DELTA..theta.=2.pi..theta..DELTA.t. The pulses typically used in ultrasound imaging contain a wide range of frequencies, so this equivalence is only approximate. Some ultrasound imaging systems use the approximate equivalence to combine time delays and phase delays to produce the desired focusing on transmit and/or receive. The process of applying time and/or phase delays to produce focused transmit and receive beams is often called "beamforming."
An ultrasound image is formed by making a series of reflection measurements in a set of desired directions. For each measurement, a focused ultrasonic wave is transmitted. Then the system switches to receive mode and the reflected ultrasonic wave is received, focused and stored. When a complete set of scan directions has been obtained, the ultrasound image is constructed and displayed, and the process then repeats for the next imaging frame. A number of such ultrasonic imaging systems are disclosed in U.S. Pat. Nos. 4,155,258; 4,155,260; 4,154,113; 4,155,259; 4,180,790; 4,470,303; 4,662,223; 4,669,314 and 4,809,184, all of which are assigned to the instant assignee.
Any factor causing a variation in pulse arrival times will produce phase variations across the transducer array, thereby reducing transducer efficiency and its ability to distinguish between on-axis and off-axis signals. For example, spatial inhomogeneities of the refraction index can lead to significant variations in propagation velocity, inducing phase delays (phase aberrations) that reduce the efficiency and the directivity of the transducer. If the sound speed is not constant, sound pulses transmitted from certain elements in the array can arrive earlier or later than expected at the desired focal point and will not properly combine with the other pulses. As a result, the net transmitted wave will not be optimally focused. Similarly, on reception, the signals on each element in the array will not be delayed optimally before summing so that the receive focusing will be degraded. If the deviations from the assumed propagation times could be measured or estimated, the ultrasound image could be improved by correcting the applied time delays for the deviations.
The human body is known to consist of many different tissues with differing sound speeds. Despite this, in medical applications the assumption of constant sound speed produces good images on many patients. However, the distribution of the various tissue types varies widely with patients, and some patients are only poorly imaged. The body wall, in particular, which includes relatively thick muscle and fat layers with sound speeds significantly different from the average sound speeds of the internal organs, can degrade the image for some patients. The solution to this problem involves correcting for phase aberrations so that transducer efficiency and directivity are restored. There would be a substantial medical benefit if the images of these patients could be improved by correcting for nonuniformity in the ultrasound propagation speed in their bodies. Such phase aberration corrections may need calculation for each separate transmit/receive direction, since the ultrasound propagation speed nonuniformities may vary significantly with beam direction. The corrections may also require calculation on a real-time basis due to patient and transducer motion in clinical applications.
A general phase aberration correction method applicable to coherent imaging systems using a sampled aperture has been proposed by Flax and O'Donnell, Phase-Aberration Correction Using Signals from Point Reflectors and Diffuse Scatterers: Basic Principles, IEEE Trans. on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 35, No. 6, November 1988. Signals emanating from a point source, or a small region of distributed scatterers, exhibit uniformity across the transducer array. Since focus exists only if the signal from a point radiator arrives at each transducer element in coincidence, the degree to which coincidence is violated is a measure of the "lack of focus" or "arrival time (phase) error." If the signal at each element is very similar, then the time offsets can be readily detected and quantified using a cross-correlation measurement between any two elements. In particular, the time offset of the peak in the cross-correlation function is a direct measure of the arrival time difference between neighboring channels. The method of Flax and O'Donnell uses only phase difference information associated with adjacent elements in a phased-array system. In the method of Flax and O'Donnell (and other prior art methods), the phase difference estimates are indirectly obtained from estimates of the correlation function associated with pairs of channel signals. Prior art phased-array imaging systems do not correct phase aberrations by using closed loop circuits, in which phase estimates are directly obtained from measured data (and not from estimated correlation functions).